1. Field of the Invention
The present invention relates generally to RF switches, and more particularly, to single-pole, multi-throw (micro-electro-mechanical) MEMS RF switches.
2. Prior Art
MEMS switches are called as such because they use electrostatic actuation to create movement of a beam or membrane that results in an ohmic contact (i.e. an RF signal is allowed to pass-through) or by a change in capacitance by which the flow of signal is interrupted and typically grounded.
In a wireless transceiver, pin diodes or GaAs MESFET's are used as switches. However, these have high power consumption rates, high losses (typically 1 dB insertion loss at 2 GHz), and are non-linear devices. MEMS switches on the other hand, have demonstrated insertion loss of less than 0.5 dB, are highly linear, and have very low power consumption since they use a DC voltage for electrostatic actuation. If the actuators are coupled to the RF signal in a series switch, then the DC bias would need to be decoupled from the RF signal. Usually, the DC current for the pin diodes in conventional switches is handled in the same way. Decoupling is never 100% and there are always some losses to the RF signal power either by adding resistive losses or by direct leakage. Another source of losses is capacitive coupling of actuators to the RF signal (especially when a series switch is closed). If high power is fed through the switch, then a voltage drop of about 10V is associated with the RF signal. That voltage is present at the RF electrode of the series switches in the open state. If these electrodes are also part of the closing mechanism (by comprising one of the actuator electrodes) that could cause the switches to close and thus limit the switch linearity (generate harmonics etc.). Usually transistor switches such as CMOS or FET suffer from non-linearity and high losses.
U.S. Pat. No. 5,619,061 to Goldsmith et al. has shown designs of MEMS switches comprised of metal and dielectric films for both capacitive coupling and ohmic contact but the metal films and the designs proposed by their invention rely on thin metal films either on top or below a beam made out of a dielectric material. A disadvantage of this type of switch is that unless the beam is made out of a single metal, there is no effective heat dissipation mechanism due to the Joule heating effect generated by the a high power RF signal that may go through.